Pretty-print keys without <> around modifiers (bug#45536)

Pretty-print keys without <> around modifiers (bug#45536)

Be consistent when pretty-printing keys: put modifiers outside <>,
thus the more logical C-M-<return> instead of <C-M-return>.

* src/keymap.c (Fsingle_key_description):
Skip modifier prefix before adding <>.
* doc/lispref/help.texi (Describing Characters): Update example.
* doc/lispref/debugging.texi (Backtraces):
* doc/lispref/minibuf.texi (Text from Minibuffer):
Use @kbd instead of @key.
* etc/NEWS: Announce the change.
* test/src/keymap-tests.el (keymap--key-description):
* test/lisp/subr-tests.el (subr--kbd): New tests.